Blender VSE - background color of video from imported image sequence
It now appears that I was wrong in suspecting this was something going wrong in the stage of rendering from image sequence to video. That is, it doesn't seem to be a Blender problem.

Timeline of my efforts:
I had viewed the rendered video with Windows Media Player, with Firefox, and with VLC player, same background color in each. With these three players giving the same result the error seemed to be in the video file itself.

For comparison I downloaded and installed Openshot video editor. Rendered a video (displayed with the same wrong background in the three players), then re-imported that video in Openshot. Then in the Openshot preview display the background color was correct: White. 

Next I transfered a Blender rendered video to another computer, played it with VLC player, took a screnshot of a still and checked the background color, came back as RGB(255,255,254). 

All of this indicates that the root of the problem is some misconfiguration of my graphics card.  Somehow display of the video goes wrong for each of the players that I used : Windows Media Player, Firefox, and VLC player. 

While I don't know how to address the misconfiguration problem, it's pretty clear now that it never was a Blender problem.